A noble metal including platinum (Pt) is known to be used as an electrode of a spark plug used in an internal combustion engine. For example, in the spark plug disclosed in Japanese Patent Application Laid-Open (kokai) No. H06-60959, a discharge member formed from platinum or a platinum-iridium alloy is bonded to an electrode base material via an intermediate member formed from a platinum-nickel alloy. A diffusion layer is formed between the discharge member and the intermediate member. This suppresses peeling or falling off of the discharge member due to thermal stress between the members.
However, in recent years, in order to further improve fuel economy, the temperature within a combustion chamber of an internal combustion engine needs to be further increased, and a spark plug needs to operate under a higher-temperature environment. Under such a high-temperature environment, since wear of a discharge member due to spark, oxidation, or the like, and peeling of a discharge member due to thermal stress or the like, are more likely to occur, wear resistance and peeling resistance need to be further improved.
For example, in the spark plug disclosed in Japanese Patent Application Laid-Open (kokai) No. H06-60959, platinum or a platinum-iridium alloy is used as a discharge member, and a platinum-nickel alloy is used as an intermediate member. However, under the above-described high-temperature environment, there is a possibility of embrittlement and decrease in thermal conductivity due to increase of elements in the diffusion layer and Kirkendall voids that occur due to progressing of interdiffusion between the discharge member and the intermediate member. In addition, for example, when platinum is used as a discharge member, crystal grains are likely to grow in platinum and intercrystalline cracking is likely to occur. Since high-temperature combustion atmosphere is likely to reach the vicinity of an interface with the diffusion layer due to the intercrystalline cracking, diffusion may progress and intercrystalline cracking may be thus increased. Therefore, peeling resistance and wear resistance are likely to decrease. When a platinum-iridium alloy is used as a discharge member, oxidation wear of iridium is likely to occur under a high-temperature environment, and the diffusion layer is likely to be embrittled due to iridium and nickel being mixed in the diffusion layer. Since iridium is reduced due to oxidation, crystal grains on the surface of the discharge member gradually grow as seen in platinum, and the crystal grains fall off as in the case of platinum. As a result, under the high-temperature environment, temperature is likely to increase in the vicinity of an interface between the discharge member and the diffusion layer, diffusion may progress, and the wear resistance and peeling resistance of a spark plug may be decreased.
The present specification discloses a spark plug that can achieve both wear resistance and peeling resistance of a spark plug under a high-temperature environment.